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Ported enclosures (aka: Bass-Reflex,
Vented) have actually been around longer than sealed designs. The ported
enclosure was patented in 1932 by A.C. Thuras. Further development since
then has defined the behavior of ported systems much more precisely. A.N.
Thiele and Richard Small are generally credited with having done the most
definitive work in this area, which is why enclosure/speaker parameters
are commonly referred to as Thiele-Small parameters.
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The coupling of a port or duct to
the air inside the enclosure allows the subwoofer system to take advantage
of the work being done by the rear of the woofer cone to reinforce the
low-frequency response. The resonant characteristics of the column of air
in a port, when installed in a given box, are adjusted by altering its
resistance to motion, which is accomplished by changing the dimensions
of the port. In some designs, instead of a port, a speaker cone with no
motor assembly or a flat diaphragm is used to achieve the same effect.
This is known as a passive radiator. The resonance of a passive radiator
system can be adjusted by altering the radiator's surface area, mass and
compliance (stiffness of suspension.)
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In a ported enclosure, there is
a delicate relationship between the volume of air in the box, the resonant
effect of the port, and the parameters of the speaker being used. When
these three factors are correctly integrated, the rear output wave of the
speaker is delayed just enough so that when it comes out of the port, it
is in relative phase with the wave being produced by the front of the speaker.
The result is constructive output from the port limited to a desired low-
frequency range. This low-frequency reinforcement is one of the big advantages
of a well-designed ported system. Using the work of the rear of the cone
in a constructive manner means that a gain in efficiency of about 3dB over
a broad band in the sub-bass range can be achieved as compared to a sealed
enclosure using the same woofer. The other big advantage is that the interaction
of the port, the enclosure and the speaker's resonant characteristics also
reduces cone motion and, therefore, distortion at higher volume levels
in the frequency range controlled by the port. The down side is that at
frequencies below the tuning of the port, the speaker gradually begins
to act as if it were not enclosed at all (more on this later.)
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The increased output combined with
reduced distortion in the "meat" of the bass range (35-60Hz) is a big reason
why many home speakers and high-power sound-reinforcement systems use ported
enclosures for low-frequency reproduction. The vast majority of recording
studios also use ported enclosures as monitors for the same reasons.
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The rules governing the behavior
and proper design of ported speaker systems are considerably more complex
than those for sealed enclosures. For this reason, it is a good practice
to follow the advice of the speaker manufacturer or an experienced enclosure
designer when it comes to designing a ported system. It is very easy to
screw up a ported box if you just guess at the size and
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length of the port or the
tuning frequency for the box. Not only will a poorly designed box sound
bad, but it can easily damage the speaker if it is played hard.
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